Direct coupled amplifier



Jim. 1, 1935. B. A. J. TEN BRINK 1,986,554

DIRECT COUPLED AMPLIFIER Filed July 3, 1931 JMIO Patented Jan. 1935 lelectrical amplifier consistinglof two 'vnves; there.

voltage is added, for the firstjvalve, to that of the DWWIQ P BarendAlbert Jan'tenBrink, The Hague, Netherlandsg assignornto HeinrichWillemBrcy, The-Hague,Netherlands r Karel De Application .iul a 1931 SerialNo. 548,664 "I11,the Netherlands July 11, 1930 The invention relatestoelectrical amplifiers and more, particularlyto direct-coupledelectrical amplifiers or so-called direct-current amplifiers,

inwhich aresistance is employed for the coue pling of two amplifyingvalveswhich resistance is not only connected with the anode of the firstvalve but also on the sam'e side with the 0f the following valve. I l ca it By. the e gpressionffeed device as hereafter used ismeantasourcegof current fromwhich tappings can be taken only by meansota resistance. connected inparallel with it.- Snch a-resistance,however, will'invariably introduce thefdrift effect, referred tohereinafter on page 2,; paragraph, 2, of the specification. The greatadvantage of the invention isthat although feed} devicesare used for,the current supply the circuit arrangement of these feed devices is suchthat resistances for tapping of suitable working voltage ion theindividual tubes are not necessary. l i

According to the invention in a directecoupled is available aconsiderably higher' lpreferably twice or more times) voltage forfthe'feedingof the first valve than for the feedingof the second valve. Inaddition to the usual feed devicdlthere maybe provided an additionalfeed device which supplies no energy to thelsecond valve but whose usualfeed device.

In order that the invention may beclearly un-- derstood andreadilycarried into effect, it will now be described more'iully withreference to the accompanying drawing, inwhich-z- Figure 1 illustratesdiagrammatically a known circuit arrangement of a direct-coupledelectrical amplifier, and

Figures 2 and 3 illustrate aiagrammaucau twc examples of an arrangementof direct-coupled amplifier in accordance with theinvention.

In electrical amplifiers-of the. kind above referred'to, the grid of thesecond .valve obviously, has the, same voltage as the anode of the firstvalve. This grid, thereforacompared with the corresponding cathode,should be negative, this being necessary for f distortionlessamplification,

so that the cathode of the second valve compared with the cathode of thefirst valve must have a stillhigher'positivevolt'age than the grid ofthe second Ivalve. This has led to the choosing in known circuits offeed voltagesfor the anode circuits of both valvesin the mannerdiagrammatically illustrated in Figure 1. In that figure two valves Iand @II are "represented which, are ied from a feed device connectedwiththe terminals A, A and generating a total voltage V which, by meansof a resistance R, can be tapped off at different points. "1 is thecoupling resistancebetween the two valves, and the terminals of the loadcircuitof the second valve are represented by 2. By virtue of the pointsof connectionof the separate anode andcathode conductors, to theresistance R of the anode feeddevice, itis, seen that essentially thesame feedvoltages. are sup plied to the two valves, which, however, havea different level. The feedvoltage of the valve II lies as a wholehigher than the reed voltageof the valve I. The filament of the-valve IIhas, compared with the lower-terminal A,jsuch a posi tive voltage thatthegridof the valve II receives the desired negative grid bias comparedto the, corresponding cathode.

' In the above-described known circuit, neither of the two valvesutilizes the whole of the voltage generated between the terminals A, A,a

circumstance which is disadvantageous for various reasons. There is,alimitation .ot the grid amplitude which is capable of amplificationwith out distortion, and moreover, the second valve is not in a positionto. give itsmaximurn output. Further, the action of the first valvewhich is already encroached uponas a result of the provision of thecoupling resistance 1. (dropof potential) is still f urtherreducedsothat the advantage of the direct'coupling for distortionlessamplification is not made full use of.

By means of the invention, the removal of these disadvantages is madepossible. The basis of the invention is formed by the recognition thatthere are various advantages in-supplying to the first valve aconsiderably higher (e. g. twice or several times'highem voltage than tothe second valve. As regards the first valve, ;.there is clearly apartof the feed voltage lost in the coupling resistance. and this part isgreater theigreater is 40 chosenthe coupling resistance with the objectof obtaining .a higher, amplification;

As now the feed voltage of ,the first valve is considerably raised, thesignificance off this loss is reduced not only for the reason that thevalve itselihas at its disposal a sufiicient voltage in order to ,beable to work under favourable condi tions, but there moreover arises thepossibility of employing a coupling element whose resistance islargefcompared with the internal resistance of the valve and which is sosuited to this valve. that the, distortionless amplification attainedreaches a very much higher value than in known circuits. The arrangementaccording to thein vention isof far reaching significance, especially ifthe first amplification stage is equipped with a screened grid valve,which in general has a considerably higher internal resistance than athreeelectrode valve.

The raising of the feed voltage of the first valve is attained,according to the invention, in that, besides the usual feed device,there is employed afeedhdevioewhose voltage; for the first valve, isadded to that of the existing feed device and which, however, suppliesno energy to the second valve. There is thereby the ,fur therpossibility of employing the whole voltage cf the usual feed device forsupplying the secondva1ye and thereby to amplify withoutdistbrtionagreate energy. I

Another and not less importantadvantag'e of the above described methodof feeding is present in that the drift effect which makes itself souhpleasantly noticeable with direct-coupled amplifiers-and which has upto the present stood the way of thefpractlical application of jsuchamp-lifier's, is completely removed. In the circuit arrangementaccording to Figure 1, there is the dini'culty'thatthecircuits of thetwo valves mutu= ally reactionaccount of'their connection with the:sainefeed device so thatthe stabilityof the I amplifier as a Whole isendangered and in conse quen e' distortionlessamplification is quite131- possible.. With-nothing to apparently disturb the state ofequilibrium vof the system, the plate current of the 'last tube beginsto drift until it iqm e e 10 i. ofliii aI d; t e ri isj'entirelyfoif thecor'rect portion of its operating on racteristici difficulty is eitheraltdg'ethr er unavoidable or avoidable only with the aid of specialcompensating. circuits. On the ether hand, with the arrangementaccording to the invention, in which use is made of additional anodefeed device which only supplies energyfto the first but not to thesecondvalv'e, this diffi:

culty no longer arises. The arrangement is stable withoutmore ado-. II

The second additionalfeed d only the firstvalve, need onl put. In adirect-current inpli w I serves mainly only for voltage 'a' season, andin consequence of this' it tali esfa much smaller current than thesecondvalve which' ha's, to supply the consumption circuit (for example, aloud speaker) I with energy. I I I I f t I I consequently, even dsregarding the remaining advantages; the efiiciency ofi he amplifying arrangement according to the inventionis consider ce, as it feedsablybetter'than that oflinown circuits which it is endeavoured to obtaina better'amplification by'raising the feed voltage. In he latter case itis, .of course,fnecessary todinie'nsion the whole feed device accordingtothe current taken by the second valve. There is thereby occasioned'ahigh continuous loss 'of energy. According to the invention, aconsiderable adi /"ance is "therefore ob tained'not only 'asregardsamplificationbut also inthe economy of the whole arrangement withsmall costs. I

Referring now'to Figures? and 3 ofthe'acc'oI r'ipanying drawing, FigureZrepresents adii ec'te' current amplifier consisting of 'two'valves withan indirectly-heated screen-grid valve I, which is connected by means ofva coupling resistance 1 withIa threeaelectrode valve II, 'i Sarje'theinput terminals, and 17, the output terminals 'of the amplifier which"are connected to the, arrangement through a transformer'lfi. t I II IForf'eeding the two valves there are provided rv a small out first va eaccording to the invention, two feed devices A and B (which aresurrounded with dotted lines) and each assumed to consist of atransformer fed from the mains, with an appropriate rectifier. The feeddevice A consists of the transformer 4 and a rectifier 6, the feeddevice B of the transformer 3 and the rectifier. 5. Thetwofeed devicesare connectedwith one another-at the point 2 and are connected to thetwo valvesin such a zmanner that there is fed to the anode circuit ofthe valve, l the sum of the two voltages, but to the anodecir'cuit ofthe valve 2, onl; the voltage of the feeel device;B, Ete ckoned from thepoint 2 outwards-,athe. twofeed devices therefore generate er gridamplitude and to obtain a greater ohtputf opposed voltages.

I A smoothing circuit is connected with the feed device A which isformed by a resistance 11 and the choked and two condensers 8 and '8 andthe other of two resistances "9 and 9' and two condensers 10 and 10''.The direct voltage of the point E is therefore again positive comparedwith that of the point D. II t FroniFigure 2 it isseen that the one endof the coupling resistance 1 inthe anode circuit of the valve I, isconnected with the point E so that between this point E and'thecathodeside of the valveI there" is moviued'a jfeed'voltage corresponding tothe of the voltages of the two feed de'vices A and B; The resistance 13and the condenser '14 connected in parallel with it in the cathodeconnecting conductor of the valve I serve in known manner to conv yautomatically to the grid of this valve the desiredv negative grid bias.

Regarding the valve II, it isseen that this receives betweenthecathodeiand the output transformer 16, a feed voltage afforded onlyand deter mined by the feed device The anode'conduc tor 18 is connectedwith the output circuit of the feed device B behind thefirstsmoothi'ngcircuit 7, 8, 8'; the smoothing jc irc'uit 9, 9', 10, 10' infiuences,therefore, only the anode current of the valve 1. The reason of thisconnection will be more fully explained below.

On consideration of the voltage distribution over the who'le amplifierarrangement, it is seen that the cathode of the valve II has a positivevoltage compared withthat or the valveI which is approximately equaltothe voltageof the fee deviceA. I I I The negative grid bias of the valveII is obtained by virtue of the fact that in the coupling resistance 1there is'produced such a drop of potential that the potentialofjthe'anode'of the valve I is somewhat lower than'that of the cathodeof the valveII or thatofthepoint of connection 2 of the two feed devices'A and B. The screen grid of the valve I is'only'fedfrorn the feeddevice A and assumes through the drop of potential in the resistance 19a potential which is again somewhat lower than the anode potential ofthis valve. In the above-mentionedcircuit the valve I as well as thevalve II receives a feed voltagewhich ensures an optimum for theamplification under the special'circumstancesdetermined for thesevalves. I I I Asthe first valve'only takes a very small curr nt rrom theseries connection of the two feed devices, in consequence of thecoupling resistance 1,'the feed device A is onlyvery lightly'loaded; it,

therefore; needs to be designed "'onlyfdr .a' small output' which isadvantageousto the emciency of.v 'the whole arrangement. Further, thedirect voltage afforded'by the feed device A, will be approximatelyequal to the peak value of thevoltw age generated in the secondarywinding of the transformer 3. By means of this high voltage;

one isiin" a position to adapt the icouplingu'esistance 1 in anappropriate manner to the valve I.

As already mentioned, there thus occurs a'high amplification, while thevalve, however, receives a sufficient voltage to enable it to work onthe kinds of valves may be employed with equal:

success.

The feed device B is provided with two smoothing circuits for the reasonthat practice has shown that thefeed current of the valve I should beaccording to Figure 2.

usually smoothed more than that of the valve II;

"This is attained in that the usual smoothing circuit '7, 8, 8' of thefeed arrangement for the end valve II is arranged with a secondsmoothing circuit 9, 9, 10, 10'. As, however, is to beseen. from Figure3, the connecting in series of the two smoothing circuits is notabsolutely necessary.

V Figure 3 represents a somewhatldifierentcircuit arrangement of thedirect-coupled amplifier according to the invention in which asimplification has been provided in the formation of the two feeddevices, whereby the cost of the arrangement is reduced. The operationotherwise agrees completely with that of the arrangement Inthe'arrangement of Figure 3, the two recti fiers 5 and 6 are connectedwith the secondary;

winding of the same transformer 4. Thus, therefore, the two feed devicesA and B are, as it were, connected with one another, whereby onetransformer is saved. The low loading of the rectifier 5 for the feedingof the valve I makes this change possible without further ado. In orderto be able to regulate within certain limits the voltage afforded bythis rectifier, a tapping-off switch is provided between the rectifierand the secondary winding of the transformer 4.

The circuit arrangement according to Figure 3 deviates further from thataccordingto Figure 2 advantage that the drop of potential arising fromthe anode current of the valve II in the choke 7 is not subtracted fromthe feed voltage intended for the valve I.

In the circuits according to the invention, it is advantageous toconnect the point of junction of the two feed current devices to earth.The advantage is thus obtained that the maximum voltage with respect toearth is reduced. This choice of earth connection is, however, not ab-On the other hand, the whole solutely necessary. The earth connectioncould be made in the usualmanner on the cathode ofzthe valve I.

'Ifqthe point of connection of the two feed devices is earthed, it isobviously necessary to insulatethe input circuit of the amplifier fromthe amplifier stages which may precede it, or from other devices, andthis may be brought about, for example, by the use of an inputtransformer. v

The supply to the cathodes of the amplifieror rectifier valves is notrepresented either in Figure 2 or in Figure 3 as this forms no part ofthe invention.

It is to be understood that the valve I may act not only as amplifierbut also as detector.

Also it is obvious that the above-described circuit arrangement, insofaras it concerns the rectifier, may be made double-acting (full waverectification). Usually such an arrangement will only come intoconsideration with very large outputs. As a result of the very highvoltages with which the arrangement according to the invention workscompared with known circuits, the

smoothing condensers employed in most cases completely suffice to bringabout a sufiicient smoothing with simple rectifiers also.

. What I claim is:--

1. A direct coupled electric amplifier comprising a first valve and asecondvalve, a high voltage feeddevice comprising a rectifier and havingits terminals connectedwith the cathode of the first valve'and theanodes of both valves respectively, an additional feed device comprisinga rectifier and having its terminals connected with the cathode of thefirst valve and the cathode of the second valve respectively, saidadditional feed device having a connection for the anode of the firstvalve through the high voltage feed device.

2. An amplifier according to claim 1, in which the point of connectionof the two feed devices is connected to the cathode of the second valve,the other terminals of the two feed devices are respectively connectedwith the anode conductors of, the two valves and with the cathodeconductor of the first valve, and smoothing circuits interposed in eachof the connections from said other terminals.

3. A direct-coupled electrical amplifier comprising two valves, a highvoltage feed device, an additional feed device, a connection between thetwo. devices, the common point of connection between the two devicesbeing connected to the cathode of one of the valves, the oppositeterminal of the high voltage feed device being connected to the anodesof the two valves, the opposite terminal of the additional feed deviceheingconnected to the cathode of the other valve, and smoothing circuitsinterposed in the connections from said opposite terminals.

4. Amplifier according to claim 1, in which the point of connection ofthe two feed devices is connected to the cathode of the second valve,the opposite terminal of the one feed device is connected to the anodeconductors of the two valves, the opposite terminal of the other feddevice is connected with the cathode conductor of the first valve,andthe feed device providing the anode conductors of the two valves withcurrent is provided with two smoothing circuits, of

which one is interposed in the connection between said feed device andthe anode conductor of the first; valve, and the other is interposed inalricde contiuetor'of the secia'fid valve'or the" Ode endiictors or:bothvalves. 1

5. A direct-coupled electrical amplifier accbrd iii-g: tdlaini 3 inwhich thefirst' valve 'h'aea screen grid connected through a-fGSiStflllGwith the" point (if 'comiec'tibn of the two? feet! devices.-

6. A direct coupled electrical amplifier accord ing to claim '3 inwhich'the high voltage feed device emdthe' additional feefi devicecomprise 9L" c'bmmen secondary winding ore; fee'd transformer and twoseparate rectifiers,one end of salidiwind-T ing' beingconnectedfio thecathode of onebf the valves-and other points df said Windingbeingconncted'to the anode 'of one rectifier and to the cathode of theother rectifier respectively;

" BAREND ALBERT JA-N TEN BRINK;

